How to Encrypt Messages with an Alphabetic One Time Pad

Using an one-time pad allows people to exchange 100% unbreakable messages, provided that the rules are properly followed. Whether exploring the world of cryptography, exchanging notes in class, or planning revolutions, using a one-time pad can allow you to communicate securely.

Steps

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    Create and maintain a one-time pad.
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    Write your message. Your message cannot be longer than the length of the one-time pad. You may want to establish abbreviations and other shorthand with your partner to ensure you can communicate effectively.
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    Create the cipher text. Use the method on which your pad is based.
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    Assume that your message is I LOVE WIKIHOW
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    For ease, you can use a Vigenere square or you can do the encryption by hand.
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    Assume our one-time pad key is: FLZYZ DLHSK LMKLI TNDXN HOEPV BNCJW
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    The first set of characters (FLZYZ) are used to start your message so that the receiver can verify that you are both using the same key.
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    To do the encryption by hand, take the clear-text letter (in this case, the letter I) and figure out it's numerical value. A=0, B=1, C=2, and continues through Z=25. The letter I would be 8 and D would be 3. It is traditional to subtract the numbers when encrypting and add the numbers when decrypting. You must agree upon which convention to use with your partner. In this case, we will use the traditional method; we would be subtracting 8 from 3, which would be 5.
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    If the result is negative, add 26. This is known has modular arithmetic.
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    Here is the overall result:
Message: I L O V E W I K I H O W
08 11 14 21 04 22 08 10 08 07 14 22
Key: D L H S K L M K L I T N
-03 11 07 18 10 11 08 10 11 08 19 13

Result: 05 00 07 03 -6 11 00 00 -2 -1 -5 -9
Modulo 26: 05 00 07 03 20 11 00 00 24 25 21 17

Cipher: F A H D U L A A Y Z V R
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    The unused portions of the key are left out of the encoded message.
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    Send the resulting message. To prevent an eavesdropper attempting to figure out the message using word length, it is often stylized into groups:
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    FLZYZ FAHDU LAAYZ VR
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Tips

  • You can add an additional number to make your cipher more complex. But this modification must be agreed upon. This is typically done when either party has to inform the other that they or the cipher has been compromised. For example, you and your friend agree to always add 10 to cipher calculations. Should the one-time pad ever get stolen or copied, or if one of you gets captured, you can send your cipher without adding 10. This would alert the other person that the message is compromised.

Warnings

  • Having a one-time pad on your person may be suspicious if you are arrested or captured. Some jurisdictions, having a one-time pad may even be considered illegal.

Article Info

Categories: Codes and Hidden Messaging